This invention relates to a damping device such as a shock absorber placed between a suspended part and an unsuspended part. For example, the suspended part may be the chassis or body of a vehicle and the unsuspended part may be the vehicle wheels.
Selection of the value of the damping force in a hydraulic shock absorber requires a difficult compromise. During a compression movement of the shock absorber, damping force must be limited so that a sudden impact which occurs when the wheel strikes a small obstacle does not cause too great an acceleration of the mass of the suspended part.
It has been proposed in U.S. Pat. No. 3,966,030 to construct a damping device to maximize damping of the oscillations of the suspended mass during compression movement until the rate of the compression movement exceeds a predetermined maximum value, as when the vehicle wheel hits a slight obstacle which causes a sudden impact on the shock absorber, at which time the damping force decreases.
The hydraulic shock absorber described in this U.S. Pat. No. 3,966,030 includes a regulating valve carried by the cylinder and operated by hydraulic fluid pressure. When it is open, the valve connects the two chambers that are on opposite sides of the main piston in the shock absorber cylinder. The operating pressure of the regulating valve is produced by the penetration of the piston rod into one of the chambers of the shock absorber cylinder, the excess fluid resulting from the reduction of the cylinder volume being retarded while flowing to a hydraulic storge space.
When the velocity of the piston rod exceeds a maximum value during a rapid compression movement, the differential pressure acting on the two faces of the regulating valve increases beyond a predetermined value and opens the regulating valve to produce a rapid reduction of the damping force. Since the hydraulic pressure exerted on the regulating valve is a function of the rate of movement of the piston during compression, the damping force decreases as a function of the axial velocity of the shock absorber piston.
U.S. Pat. No. 3,966,030 also discloses that the shock absorber may have an elastic member which is connected to the vehicle. When the acceleration of the rod is very great, the elastic member is first compressed to increase the volume of the parts immersed in the cylinder. This regulates the regulating valve in the manner described above before the damping force reaches its maximum value.
A particularly advantageous embodiment of a damping device using the teachings of the U.S. Pat. No. 3,966,030 and its French equivalent Patent No. 71 00 893 was also the subject of U.S. Pat. No. 4,054,277 and its equivalent French Pat. No. 74 34 286. In this embodiment, the regulating valve which is always mounted stationarily in the cylinder and whose opening causes a rapid reduction of the damping force, is subjected to the action of a substantially constant reference pressure. One of the faces of the regulating valve provides a surface which forms a wall of a gas-filled reference chamber. This reference chamber can be closed and isolated or opened to the atmosphere, the reference pressure remaining exactly constant. Alternatively, the reference chamber may be connected to the gas-filled part of the hydraulic fluid storage space to provide a pressure balance which is independent of operational variations due to thermal expansion. In all cases, the regulating valve is operated in response to the increase of the pressure of the hydraulic fluid in an intermediate space located between the first chamber of the shock absorber cylinder and the second chamber which receives the piston rod.